The present disclosure relates to a fuel injection control apparatus for an internal combustion engine and a method for operating the fuel injection control apparatus.
Japanese Laid-Open Patent Publication No. 2016-217324 describes one example of a fuel injection control apparatus for an internal combustion engine. The device includes a low pressure fuel pump that pumps fuel from a fuel tank, a low pressure fuel passage through which the fuel discharged from the low pressure fuel pump flows, a high pressure fuel pump that pressurizes the fuel supplied from the low pressure fuel passage, a high pressure fuel passage through which the fuel pressurized by the high pressure fuel pump flows, and a fuel injection valve that injects the fuel of the high pressure fuel passage.
The device further includes a regulator connected to the low pressure fuel passage. When fuel pressure of the low pressure fuel passage is higher than or equal to a predetermined pressure, the regulator is actuated to return the fuel from the low pressure fuel passage into the fuel tank so that a further increase in the fuel pressure is limited. In other words, the regulator adjusts the fuel pressure of the low pressure fuel passage.
To reduce energy consumption and improve fuel economy of internal combustion engines, a fuel injection control apparatus that controls a driving amount of a low pressure fuel pump to adjust fuel pressure of the low pressure fuel passage has been developed. In such a device, the driving amount of the low pressure fuel pump is calculated, for example, based on the sum of a feedforward correction amount and a feedback correction amount. The feedforward correction amount is calculated to increase as a request injection amount of a fuel injection valve increases. The feedback correction amount is calculated based on a deviation of fuel pressure of the low pressure fuel passage from its target value. The fuel pressure of the low pressure fuel passage is adjusted by controlling the low pressure fuel pump at the driving amount calculated as described above.
In a device including a low pressure fuel pump and a high pressure fuel pump, a high pressure target value, which is a target value of fuel pressure of the high pressure fuel passage, may be changed by a running state of the internal combustion engine and the like. For example, when the high pressure target value is increased, the high pressure fuel pump is driven to increase the fuel pressure of the high pressure fuel passage to the high pressure target value. In this case, the high pressure fuel pump is driven to supply fuel corresponding to the increased amount of the high pressure target value from the low pressure fuel passage to the high pressure fuel passage. At this time, the feedback correction amount does not increase until the absolute value of the deviation of the fuel pressure of the low pressure fuel passage from its target value is increased to a certain level. Therefore, when the low pressure fuel pump is controlled at the driving amount based on the sum of the feedforward correction amount and the feedback correction amount calculated as described above, the amount of fuel discharged from the low pressure fuel pump will not be increased until the absolute value of the deviation of the fuel pressure of the low pressure fuel passage from its target value is increased to a certain level. Consequently, the fuel pressure of the low pressure fuel passage decreases.
Additionally, the capacity of recent high pressure fuel passages has been increasing. As the capacity of a high pressure fuel passage increases, the low pressure fuel passage will supply a greater amount of fuel to the high pressure fuel passage to increase the fuel pressure of the high pressure fuel passage. In other words, as the capacity of a high pressure fuel passage increases, a greater amount of the fuel pressure of the low pressure fuel passage will be decreased by an increase in the high pressure target value. A significant decrease in the fuel pressure of the low pressure fuel passage may allow vapor to be generated in the low pressure fuel passage.
In a device that adjusts fuel pressure of a low pressure fuel passage by driving a low pressure fuel pump, there is room for improvement for limiting decreases in the fuel pressure of the low pressure fuel passage when a high pressure target value is increased.